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How Radiator Cap Pressure Affects Cooling

2013 August 20
by therealcoleq

Flex-a-lite-radiator-capOne of the Monday Tech Quizzes that we posted the Flex-a-lite® Facebook page was about radiator caps, and whether or not the pressure of the cap might help solve an overheating problem. As we tried to formulate a short answer for this question, we were reminded of how complicated the topic really is. So we decided a post on our blog would be the best way to provide useful information on the topic.

First, the radiator cap is an important part of the cooling system. It is designed to hold a certain amount of pressure inside the system, and to release the pressure when it exceeds a pre-determined level. Why do you want pressure in the cooling system? Actually, you don't get a choice. As the coolant heats up, it expands. Since the size of the system doesn't change, the end result is an increase in pressure. However, the positive impact is that the boiling point of the coolant increases as the pressure increases. Straight water at atmospheric pressure boils at 212 degrees Fahrenheit. A mix of 50/50 coolant and water boils at around 220 degrees. That same mix at 16 pounds per square inch (psi) of pressure boils at approximately 250 degrees. And the boiling point increases 2-3 degrees for each additional psi of pressure in the system.

So the over-simplified answer to the tech quiz is that, yes, a higher pressure radiator cap can help prevent overheating. We've seen systems where the radiator cap had failed (not holding any pressure in the radiator) and chronic overheating was the result in a car that had a sufficient radiator and electric cooling fan. Simply replacing the radiator cap solved the problem. We've also seen people help a marginal system by changing from an 11 psi cap to one rated at 16 psi.

Why not just jump to the highest psi caps available? Because cooling systems are designed for specific pressure. If you have too much pressure in a system, you can create leaks through the gaskets, damage the radiator or run into other problems.

It is important to have a properly functioning radiator cap, and we recommend that you install a new radiator cap when you install a Flex-a-fit® aluminum radiator from Flex-a-lite®. Use one with the psi rating that the rest of your cooling system was designed for. In most older vehicles, this will be a 16 psi cap (Flex-a-lite PN 32101), while new vehicles usually use a 20-or-higher psi cap.

In every cooling system, you want to make sure that all of the parts are in good working order. Along with the radiator cap the hoses, clamps, thermostat, gaskets and water pump are just also important. Clean and fresh coolant is key as well. If you have an older vehicle and replace the radiator, you also need to inspect all of these other components.


2 thoughts on “How Radiator Cap Pressure Affects Cooling”

  • Lynn Mc Lemore
    Lynn Mc Lemore September 3, 2013 at 4:50 am

    I bought the flexalite model 264 for my 2006 Dodge 3500 5.9l. The fans worked perfect on flat ground pulling a 12000# 5th wheel. The day we started up a steep grade in New Mexico going to Ruidoso the truck over heated. We regrouped and put the OEM equipment back on and have had no problems with the steep grades. I would not recommend this product on a Dodge diesel truck pulling a load in the mountains.

    • therealcoleq
      therealcoleq September 3, 2013 at 12:52 pm

      We're sorry to hear about your experience. If you want to explore causes of this with our tech team and look for a solution, please contact them at 253-922-2700.

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